The present invention relates to a thread forming tap, and more particularly, to a thread forming tap which is capable of reducing tapping torque as well as improving durability by avoiding an increase of an apparent area of margins of external thread that causes interference with a grinding stone.
Typically, a tap is used to form internal threads. Taps are mainly classified into two types, a cut thread tap for cutting the surface of a prepared hole (hole before threading), and a thread forming tap for performing plastic deformation on the surface of the hole before threading. The cut thread tap, however, requires an elimination process due to the chips generated by the tapping, and the finished surface comes out rough. On the other hand, in the thread forming tap, no chips are generated during the tapping process, and the finished surface of the internal thread comes out smooth as well as durable, resulting in a wide range of use.
A conventional thread forming tap is shown in FIGS. 6A and 6B. As shown in a front view of FIG. 6A, the tip of the conventional thread forming tap 100 is established with an external thread 101 for rolling internal threads. A taper shaped chamfer 101a and a full thread part 101b are established on the external thread 101, and ridges are formed along the helix with a predetermined lead angle. Further, as shown in FIG. 6B, the external thread 101 has a square shape in cross-section, where four radially projected margins 102 and four reliefs 103 linked to the respective margins 102 are alternately established on the external thread 101 in the direction of the screw thread, i.e., along the helix of the ridges. In the example of FIG. 6A, each margin 102 is aligned in parallel with an axis O of the thread forming tap 100.
When forming an internal thread, the chamfer 101a of the thread forming tap 100 is spirally advanced on the surface of the prepared hole on a workpiece. By this spiral movement, the margins 102 on the chamfer 101a bite into the prepared hole on the workpiece, allowing the plastic deformation (plastic flow) while proceeding on the surface of the hole, thereby creating the internal thread. This internal thread is then finished by the margins 102 on the full thread part 101b which helically advances as well.
In the conventional thread forming tap 100, however, as shown in FIG. 7, the ridges (crests and roots) on the external thread 101 are at a lead angle xcexa, unlike the margins 102 which are parallel with the axis O. Therefore, during the grinding process for producing the thread forming tap, in which a grinding stone having ridges on the outer surface thereof proceeds along the lead of the ridges to grind a screw thread onto the external thread 101, an interference is occurred by the grinding stone with the margins 102, resulting in the increase of an apparent width (area) of the margins 102. This increase in the apparent width (area) causes a rotational resistance during tapping, i.e., an increase in tapping torque, resulting in an increase in the rolling load which also causes breakage of the margins 102.
Therefore, the object of the present invention is to solve the above noted problems involved in the conventional technology by providing a thread forming tap with low tapping torque and high durability that is capable of preventing the increase of the apparent area of the margins caused by the interference with the grinding stone.
In order to achieve this object, the thread forming tap in the first aspect of the present invention comprises an external thread established with radially projecting margins and reliefs smaller in diameter than that of the margins. The margins and the reliefs are alternately linked to one another on the external thread in a screw thread shape. When forming internal threads, the margins on the external thread bite into the surface of a prepared hole of a workpiece, where plastic deformation is performed. The margins are aligned helically in a direction opposite to a direction of a helix of the external thread.
In the thread forming tap in the second aspect of the present invention, the external thread has a lead angle xcex1. The margins on the external thread have a helix angle xcex2, where the helix of the margins is in the opposite direction of the helix of the external thread. Further, the helix angle xcex2 of the margins is larger than 0xc2x0 but smaller than 10xc2x0 or 4xcex1.
In the thread forming tap defined in the third aspect of the present invention, oil flutes established on the external thread are directed either in parallel with or diagonal to the axis of the external thread. The oil flutes are extended in a straight line over either a part of or the entire external thread.
In the thread forming tap in the fourth aspect of the present invention, the oil flutes established on the external thread is inclined in the same manner as the margins of the external thread. The oil flutes have a helix angle xcex2, where the helix is in the opposite direction of the helix of the external thread. The oil flutes are extended over either a part of or the entire external thread.